Environmental problems such as global warming are in the news on a global basis, so that development in the technique of improving fuel efficiency of an internal combustion engine for reducing CO2 in atmosphere is a big challenge. As part of it, reduction in friction loss of a sliding member used for an engine or the like is in demand. In view of the demand, in recent years, the technique on material, surface process, and reforming technology of a sliding member having excellent wear resistance and scuffing resistance and capable of maximally expressing the effect of reducing frictional force is being developed.
To improve energy efficiency of an apparatus using a cylinder such as fuel consumption of an internal combustion engine, it is effective to reduce friction loss. Particularly, between a piston ring which reciprocates and the bore surface of a cylinder, reduction in friction is effective. To reduce the reciprocation friction, reduction in surface roughness of the bore surface of the cylinder is valid means. However, when surface roughness is too small, lubricating oil held in the bore surface hardly exists, and it causes an inconvenience such that scuffing resistance deteriorates. To improve the scuffing resistance, in Patent Document 1, a cylinder liner is formed so that the surface roughness of the bore surface of the cylinder liner increases from the top dead center side toward the bottom dead center side of the piston. However, in the Patent Document 1, the surface roughness at the bottom dead center and the stroke center region is large. Consequently, there is an inconvenience such that reciprocation friction increases.
Patent Document 2 discloses a technique of reducing reciprocation friction between a piston ring and a cylinder liner by forming depressions in the bore surface of the cylinder liner. In the Patent Document 2, the cylinder liner is divided into a plurality of regions in the axial direction of the cylinder according to variations in sliding speed and the shapes of the depressions are made different by regions, thereby enhancing the effect of reducing the reciprocation friction. However, in the Patent Document 2, a number of circular depressions are formed at least in portions around sliding ends at which a sliding member turns back of a sliding surface. Since the sliding speed decreases when the piston reaches the top dead center and the bottom dead center, in the case where the depressions are formed in the portions near the sliding ends, there is inconvenience such that the oil film becomes thin, metallic contact tends to occur, and friction increases.    Patent Document 1: Japanese Patent Application Publication No. 8-200145    Patent Document 2: Japanese Patent Application Publication No. 2007-46660